The disclosure of the invention relates to subject matter contained in U.S. Pat. Nos. 3,663,338, 3,586,142 and 3,541,929 and Patent Application Serial Nos. 198,282, filed Nov. 12, 1971, and 382,607 filed July 25, 1973. All patent documents are assigned to the assignee of the present application and their disclosures, by reference thereto, are incorporated herein.
Of the above noted prior art that which is deemed most relevant to the present application is U.S. Pat. No. 3,541,929. Reference thereto will reveal that it discloses controls and mechanisms operated thereby for interrupting the bag producing function of a thermo-plastic bag making machine. Specifically, the system includes a pair of web feeding rolls, sometimes and more usually referred to as draw rolls, between which a web of thermo-plastic material is disposed. The draw rolls are connected to and driven by a shaft mounting a clutch and a brake which are sequentially energized and de-energized to impart intermittent rotation to the draw rolls. The period of time allocated for draw roll rotation determines, with respect to side weld bags, the width of the bag and, with respect to bottom weld bags, the length of the bag. At the time that the draw rolls are momentarily stopped, a reciprocating heated seal bar descends to sever and seal the web material and in doing so establishes pressure contact with an underlying platen or seal roll. If the machine is arranged to produce bottom weld bags the seal roll is replaced by a rigid stationary heated bar since the seal requirement for bottom weld bags oftentimes differ substantially from side weld bags. All known universal bag machines include, as part of their control systems, a cycle counting device which may be set to produce a certain number of bags in succession. After the pre-determined number of bags have been made, the cycle counting mechanism conditions the circuit to automatically interrupt draw roll operation in order to prevent feeding of the web. Simultaneously with the interruption of web feed the seal bar is restrained or prevented from making contact with the web.
One of the primary objectives in present day bag machine design is to minimize the time required to effect interruption of the draw roll operation and disabling the seal bar so that it does not make contact with the leading edge of the arrested web. An acceptable time to perform these functions is one bag making cycle. Known machinery, including machinery made in accordance with the above identified patents and patent applications, operates reasonably well up to about 200 bag machine cycles per minute. Requirements for higher speeds, for example 300 bags per minute, renders bag machine interrupt systems, constructed in accordance with the above mentioned prior art, inadequate since the time constants of the operating controls, such as pneumatic cylinders and the inertia of certain machine elements, require more than 1/5 of a second (at 300 bags per min. 5 bags are produced per second) to effect interruption of the draw rolls and the seal bar.